Sodium hydroxide and chlorine that are important as industrial starting materials are mainly produced by an electrolytic soda process. This electrolysis process shifted to an ion-exchange membrane process using an activated cathode involving small overvoltage through a mercury method using a mercury cathode and a diaphragm process using an asbestos diaphragm and a soft iron cathode. This shift decreased power unit for production of 1 ton of sodium hydroxide to 2,000 kWh. The activated cathode includes a cathode obtained by dispersing ruthenium oxide powder in a nickel plating bath and undergoing composite plating, and cathodes obtained by using nickel plating containing a second component such as S or Sn, NiO plasma spray coating, Raney nickel, Ni—Mo alloy, Pt—Ru substituted plating, or a hydrogen storage alloy for imparting durability to reverse current (Electrochemical Hydrogen Technologies, p. 15–62, 1990, H. Wendt; U.S. Pat. No. 4,801,368; J. Electrochem. Soc., 137, 1419 (1993); and Modern Chlor-Alkali Technology, Vol. 3, 1986). JP-B-6-33481 and JP-B-6-33492 teach that a mixed catalyst of cerium and a noble metal is durable to contamination with iron. Recently, an electrolytic cell that can increase current density for the purpose of increasing production ability and decreasing an investment cost is now under development in an ion-exchange membrane process. Development of a low resistance membrane enables large current to apply.
DSA which is an anode has operating results in a current density up to 200–300 A/dm2 in a mercury method. However, there are no actual results in such DSA regarding life and performance when used as a cathode in an ion-exchange membrane process, and further improvement is demanded. It is important for use as a cathode to be low overvoltage, to be a membrane not impaired by contacting with a cathode, and to be low contamination with, for example, metal ions from a cathode. Thus, this makes it difficult to use the conventional electrodes (having large surface unevenness and low mechanical strength of a catalyst layer). To realize a new process, it is indispensable to develop an activated cathode having high performance and also sufficient safety even under the above electrolysis conditions.
In the recent electrolytic soda process using an activated cathode most generally conducted, a cathode is arranged so as to contact with a cathode side of a cation-exchange membrane, or to have a gap of 3 mm or lower from an ion-exchange membrane. Water reacts in a catalyst layer of the cathode to form sodium hydroxide. Anodic reaction and cathodic reaction are as follows, respectively.2Cl−=Cl2+2e (1.36V)2H2O=2OH−+H2 (−0.83V)
Theoretical decomposition voltage is 2.19V
However, where the conventional cathode is operated at a large current density, there are some large problems, for example, as follows.
(1) Part of a substrate (nickel, iron or carbon component) dissolves and peels due to deterioration of an electrode, and such a component migrates into a catholyte, a membrane or an anode chamber, resulting in deterioration of product quality and deterioration of electrolysis performance.
(2) Overvoltage increases with increasing a current density, resulting in decreasing energy efficiency.
(3) Distribution of gas bubbles in a cell increases with increasing a current density, resulting in causing distribution in concentration of sodium hydroxide formed. As a result, solution resistance loss of a catholyte increases.
(4) Where operating conditions are severe, an amount of impurities (sulfur, iron or the like) effused from a cell constituting material increases, resulting in contamination of an electrode.
It is expected that a constitution that a cathode is arranged so as to closely contact with an ion-exchange membrane (zero gap) can decrease voltage, and such a constitution is desirable. However, this constitution has the possibility that a membrane is mechanically broken by a cathode having a rough surface. Thus, there has been the problem to use the conventional cathode at high current density and under zero gap condition. A cathode using a noble metal as a catalyst has conventionally been proposed. Such a cathode may be satisfactory in performance. However, there is the problem in cost, and it is essential to decrease the amount of the catalyst used. In this case, thickness of the catalyst layer is small, so that a substrate is liable to peel by dissolution. Thus, further improvement is still required.